API for DayOfWeek enum

    /** Accessor to the abbreviation for this day.
     * @return char the one-letter abbreviation
     */
    public char letter()

    /** Accessor to a List of days, in order.
     * @return List<DayOfWeek> list of the days.
     */
    public static List<DayOfWeek> getDays()

    /** Get the day corresponding to the specified letter.
     * @param letter an abbreviation for a day.
     * @return DayOfWeek the day which matches the given letter.
     */
    public static DayOfWeek toDayOfWeek(char letter)

    /** Get the day following this one.
     * @return DayOfWeek day that follows this one in the defined sequence.
     */
    public DayOfWeek next()